In cellular networks for wireless communication, interference often occurs in a cell caused by uplink signals transmitted from User Equipments, UEs in nearby located cells, which is a well-known problem. In such a network, the available frequency bandwidth is limited and in order to provide capacity for communications in the network having multiple cells, uplink radio resources in frequency and time must be shared in two or more cells in a way so as to not disturb the communication for one another. In this context, cells that are located near a serving cell are generally referred to as “neighboring cells” and uplink transmissions by UEs in neighboring cells may potentially disturb transmissions by UEs in the serving cell, and vice versa, thus causing interference at the receiving radio node. The term “radio node” will be used here to represent any node in a communication network that is capable of communicating with UEs by receiving uplink radio signals transmitted from the UEs. Other terms than radio node commonly used in this field include base station, node B, evolved node B (eNB), base transceiver station, and so forth.
This disclosure is relevant for cellular networks using any of the following radio access technologies: Orthogonal Frequency Division Multiplexing (OFDM), Single Carrier-Frequency Division Multiple Access (SC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Time Division Multiplex (TDM), and Frequency Division Multiplex (FDM). Further, radio resources defined by frequency and time will be referred to as “resources” for short in the following description. Typically, resources for radio transmission can be defined by any combination of frequency bands and time intervals. In systems of Long Term Evolution, LTE, the resources are sometimes referred to as Physical Resource Blocks, PRBs, defined by frequency and time interval.
A general problem in cellular networks is that the performance in communications may be degraded and the network capacity may also be reduced, due to interference when the same time-frequency resources are reused for uplink transmissions in multiple nearby cells. This problem is common for networks with a so-called heterogeneous cell configuration where the coverage areas of large cells, e.g. macro cells, completely or partly overlap with the coverage areas of smaller cells, e.g. pico cells, micro cells and femto cells. These cell configurations thus form a layered or hierarchical cell structure and a simplified example of how interference can occur in such a heterogeneous cell configuration is illustrated in FIG. 1.
In FIG. 1, a large macro cell A is covered by a radio node 100 and a small pico cell B located within cell A is covered by another radio node 102. Typically, UEs connected to and served by a radio node of a macro cell need to use high output power for their uplink transmissions due to the often large distance to the serving radio node, while UEs connected to and served by a radio node of a pico cell, a micro cell or a femto cell only need to use low output power for their uplink transmissions thanks to the much shorter distance to the serving radio node.
In this example, a UE 104 is connected to the radio node 100 of the macro cell A and transmits an uplink signal denoted x with high power to the radio node 100. This signal x is also received as an interfering signal denoted x′ at the radio node 102 of the pico cell B. Since the UE 104 is located substantially closer to radio node 102 than to the radio node 100 in this case, the interfering signal x′ will be received with high signal strength at radio node 102 and therefore cause high interference with any other uplink signals in the pico cell B if transmitted on the same resource in frequency and time. Any UE connected to the radio node 102 only needs to transmit signals with low power which are therefore easily interfered by the stronger signal x′ from UE 104. It is thus a problem that uplink signals transmitted in large cells may often cause interference to uplink transmissions in small cells. In this context, the terms “large” and “small” cells should be understood to be relative in terms of inter-cell interference, and e.g. a macro cell is typically a large cell compared to a micro, pico or femto cell, while a micro cell may also be regarded as a large cell compared to a pico cell, and so forth.
In the following, a cell where uplink transmissions of high output power may cause inter-cell interference will be referred to as an “aggressor cell” which is typically but not necessarily a macro cell or similar, while a cell where uplink transmissions of low output power may be disturbed by inter-cell interference will be referred to as a “victim cell” which could be a micro, pico or femto cell or similar. Correspondingly, the radio node of the aggressor cell will in this description be called the “aggressor radio node” and the radio node serving the victim cell or receiving measurement signals transmitted in the victim cell will be called the “victim radio node”. It should be noted that the latter victim radio node may be any radio node receiving radio signals potentially subjected to interference, e.g. an eNB or base station to which UEs in the victim cell are connected and transmit such signals, or other radio node, e.g. a Location Measurement Unit LMU, that just performs measurements on uplink signals transmitted from UEs in the victim cell without actually serving the victim cell.